Primary Sources:

These are materials, which are original in  nature. They  usually take the form of a journal article, a  monograph, a dissertation,  a report or a patent or documents in archives. These mate-vials are also referred to as primary sources as they have not passed through any filtering  mechanism like  condensation, interpretation or evaluation and are the original work of the author. The primary sources are very large in number and can be usually accessed with the help of some secondary source like, a bibliography, an indexing service, or an abstracting service. 

Secondary Sources:

A secondary source is one that gives information about a primary source. The original information is selected, modified and rearranged in a suitable format for the purpose of easy location by the users, e.g., indexes, abstracts,bibliographies, yearbooks, encyclopaedias, gazetteers, who's who. An autobiography is a primary source whereas a biography written by other person is a secondary source. 

Tertiary Sources:

The tertiary sources consist of information that is a distillation and collection of primary and secondary sources. These include lists of all kinds of sources of reference works like the encyclopaedias, dictionaries, yearbooks, bibliography of 'bibliographies, almanacs, etc. By classifying and defining reference sources, we can get only an indication of their relative currency and the relative accuracy. Their main purpose all the time remains the same, that is to match a question/query of the user with a suitable answer from any of the sources.